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Extreme Morphological and Ecological Homoplasy in Tropical Salamanders

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Extreme Morphological and Ecological Homoplasy in Tropical Salamanders Extreme morphological and ecological homoplasy in tropical salamanders Gabriela Parra-Olea* and David B. Wake† Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, CA 94720-3160 Contributed by David B. Wake, April 25, 2001 Fossorial salamanders typically have elongate and attenuated We analyzed sequences of mtDNA of many tropical bolito- heads and bodies, diminutive limbs, hands and feet, and extremely glossines, including all recognized genera, and determined that elongate tails. Batrachoseps from California, Lineatriton from east- Lineatriton and Oedipina are much more closely related to other ern Me´xico, and Oedipina from southern Me´xico to Ecuador, all taxa than to each other (3, 4). Not only was Lineatriton deeply members of the family Plethodontidae, tribe Bolitoglossini, resem- nested within the large, mainly Mexican genus Pseudoeurycea, ble one another in external morphology, which has evolved inde- but populations of Lineatriton from different parts of its geo- pendently. Whereas Oedipina and Batrachoseps are elongate be- graphic range were more closely related to different species of cause there are more trunk vertebrae, a widespread homoplasy Pseudoeurycea than to each other. Here we analyze molecular (parallelism) in salamanders, the genus Lineatriton is unique in data for 1,816 bp of mtDNA derived from three genes, reject the having evolved convergently by an alternate ‘‘giraffe-neck’’ de- monophyly of Lineatriton, and support an extraordinary case of velopmental program. Lineatriton has the same number of trunk homoplasy in a putative species that previously has been con- vertebrae as related, nonelongated taxa, but individual trunk sidered to be extremely specialized, and unique, in both mor- vertebrae are elongated. A robust phylogenetic hypothesis, based phology and ecology. on sequences of three mtDNA genes, finds Lineatriton to be deeply nested within a clade characterized by generalized ecology and Materials and Methods morphology. Lineatriton lineolus, the only currently recognized We collected specimens of all populations reported herein and taxon in the genus, shows unanticipated genetic diversity. Sur- preserved either frozen tissues or placed small pieces of tail prisingly, geographically separated populations of L. lineolus are tissue in 95% ethanol. The in-group includes eight samples of L. not monophyletic, but are sister taxa of different species of the lineolus from central western Veracruz, Me´xico, and four sam- morphologically generalized genus Pseudoeurycea. Lineatriton, ples from the Los Tuxtlas area of coastal southeastern Veracruz. long thought to be a unique monospecific lineage, is polyphyletic. The first of sequential out-groups includes samples of 15 species Accordingly, the specialized morphology of Lineatriton displays of Pseudoeurycea from Me´xico. More distant out-groups were homoplasy at two hierarchical levels: (i) with respect to other selected from other members (Batrachoseps and Thorius)ofthe elongate lineages in the family (convergence), and (ii) within what plethodontid tribe Bolitoglossini. Because of the surprising is currently recognized as a single taxon (parallelism). These evo- results, several populations of the in-group and Pseudoeurycea lutionary events are of adaptive significance because to invade the were collected on repeated field trips and sequenced indepen- lowland tropics salamanders must be either arboreal or fossorial; dently. DNA was extracted following standard techniques (4). the repeated evolution of elongation and attenuation has led to We obtained 544 bp of the large 16S subunit ribosomal mtDNA multiple lowland invasions. (5), 569 bp of the cytochrome b gene (6), and 709 bp of the ND4 gene (7). Amplification was done via the PCR (8) using the ropical plethodontid salamanders form a monophyletic clade primers 16Sar and 16Sbr (9) for 16S, MVZ15 and MVZ18 (10) Tof more than 200 species that display diverse body forms, for cytochrome b, and ND4 (7) and MVZ112 (3) for the ND4 including extreme elongation accompanied by limb reduction gene. Following standard techniques (4), cycle-sequencing prod- associated with fossorial habits (1). All lowland tropical pleth- ucts were purified by using ethanol precipitation and separated odontids are either fossorial or arboreal, and fossoriality has by electrophoresis on a 6% polyacrylamide gel using an ABI 377 been an important factor in the invasion of lowlands by these DNA sequencer (Applied Biosystems). Sequences were read more typically cool-adapted upland animals. The fossorial Oe- from both strands and aligned to each other by eye in the dipina has many species that range south and east of the Isthmus program SEQUENCE NAVIGATOR version 1.0.1 (Applied Biosys- of Tehuantepec, from southern Me´xico into northwestern South tems). Sequence divergences were estimated by using the America; these differ from all other tropical plethodontids in Kimura two-parameter distance (11). Phylogenetic analyses having 18–22 rather than an invariable 14 trunk vertebrae. The were performed by using PAUP 4.0b2a (D. Swofford, Smithsonian other fossorial lineage, the equally attenuate and elongate Institution, Washington, DC). Parsimony analysis used heuristic Lineatriton lineolus, is found only in restricted parts of eastern searches by stepwise random addition of taxa, with 20 replica- Me´xico, north of the Isthmus of Tehuantepec. Oedipina evolved tions and tree bisection and reconnection branch swapping with elongation from a more generalized morphology by increasing the MULPARS option in effect. Zero-length branches were col- the number of body segments, and hence vertebrae, a develop- lapsed. Both the consistency index (12), and retention index (13) mental mechanism that has evolved repeatedly within were calculated. We used two character weighting schemes: salamanders as a whole, and even within the clade (Bolito- equal weighting for all codon positions, and differential weight- glossini) to which the tropical species belong. Lineatriton,in ing in which we downweighted third codon position transitions contrast, is unique among all salamanders in having evolved an entirely different developmental mechanism. Its trunk vertebrae Data deposition: The sequences in this paper have been deposited in the GenBank database number only 14, the same as in all other tropical salamanders (accessions nos. AF379643–AF379681 and AF380754–AF380831). except for Oedipina, but the individual vertebrae are narrow and *Present address: Museum of Comparative Zoology, Harvard University, Cambridge, MA elongate. As a consequence, the external form of L. lineolus and 02138. its adaptation for fossorial existence are so similar to those of †To whom reprint requests should be addressed at: Museum of Vertebrate Zoology, 3101 species of Oedipina that it was included in the genus Oedipina Valley Life Sciences Building, University of California, Berkeley, CA 94720-3160. E-mail: until 1950 (2). [email protected]. 7888–7891 ͉ PNAS ͉ July 3, 2001 ͉ vol. 98 ͉ no. 14 www.pnas.org͞cgi͞doi͞10.1073͞pnas.131203598 Downloaded by guest on September 27, 2021 to a ratio of 1:0.16 transversion͞transition. The transversion to transition ratio was estimated by maximum likelihood analysis. Neighbor-joining reconstructions (14) used Kimura two- parameter distances. Maximum likelihood analysis used the heuristic algorithm, empirical base frequencies, and the Hase- gawa-Kishino-Yano (15) model of character evolution. Decay indices and bootstrap values in excess of 50% are reported (1,000 pseudoreplicates for parsimony analysis with heuristic search and simple addition of taxa). Morphological comparisons are based on study of proportions of preserved individuals, cleared and stained skeletal prepara- tions, and radiographs of preserved specimens. Results The three genes studied each display extensive differentiation among populations of L. lineolus, with Kimura two-parameter distances as great as 18% (cytochrome b), 16% (ND4), and 5% (16S). These levels are much greater than typically found within other species of bolitoglossine salamanders (e.g., ref. 4), or other taxa (16). Independent and combined data analyses are consis- tent in finding mitochondrial para- or polyphyly of Lineatriton. Mitochondrial haplotypes do not form a monophyletic group but rather fall out in different parts of the mtDNA gene tree. A maximum parsimony analysis of a combined data set using equal weighting produced 80 equally parsimonious trees (L ϭ 2,183 steps; 545 characters were parsimony informative; consistency index ϭ 0.478, retention index ϭ 0.763). In this analysis (Fig. 1), populations of L. lineolus are deeply nested within Pseudoeury- cea, and samples of Lineatriton are not monophyletic. Samples of L. lineolus from southeast Veracruz (population 1, Los Tuxtlas) form a clade that is sister to the closely related Oaxacan species Pseudoeurycea werleri and Pseudoeurycea mystax. A well sup- ported clade consisting mainly of samples of Pseudoeurycea leprosa is the sister taxon of a clade that includes samples of L. lineolus from central western Veracruz (populations 2 and 3, Cerro Chicahuaxtla and Barranca de San Miguel). We recover two well supported clades (clade 1, bootstrap value 96%, decay index 10, and clade 2, bootstrap value 77%, decay index 4; Fig. 1), each containing Lineatriton, and these clades are deeply Fig. 1. (Left) Consensus tree of maximum parsimony analysis, combined data nested within Pseudoeurycea. When all samples
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